The invention relates generally to drying of plant material and, more specifically, to a novel, non-obvious process for drying of plant material to maintain a high content of a labile bio-active molecule.
Rosmarinic acid (RA) is an ester of caffeic acid and 3,4-dihydroxyphenylacetic acid. It is also a secondary metabolite of various plant species including those of Lamiaceae. Spearmint (Mentha spicata L.) is particularly known as a major source of carvone-rich essential oil for perfumery and flavoring industries and is grown worldwide. It is a fast growing perennial that can biosynthesize significant amounts of RA and other phenolics when selected to do so (Fletcher R S, McAuley C and Kott L S. 2005a. Novel Mentha Spicata clones with enhanced rosmarinic acid and antioxidant activity. Proc. WOCMAP III, Vol. 6: Traditional Medicine and Nutraceuticals Ada Horticulture. 6S0, ISHS. pp 31-40 SA-08-06337; Fletcher R S, McAuley C and Kott L S. 2005b. Heat stress reduces the accumulation of rosmarinic acid and the antioxidant activity of Spearmint (Mentha spicata L.). Journal of Science of Food and Agriculture 85:2429-2436 SA-09-06343). There is an interest in developing products based on the more polar RA extracted from spearmint that will likely have greater antioxidative efficacy than carnosic acid in beverages, sauces, and emulsions. In addition, this molecule is known to have unique properties including antiviral, antibacterial, and anti-inflammatory activities (Mazumder A, Neamati N, Sunder S, Schulz J, Pertz H, Eich E, and Pommier Y. 1997. Curcumin analogs with altered potencies against HIV-1 integrase as probles for biochemical mechanisms of drug action. Journal of medical Chemistry. 40:3057-3063; Szabo E, Thelen A and Paterson M. 1999. Fungal elicitor preparations and methyl jasmonate enhance rosmarinic acid accumulation in suspension cultures of Coleus Blumei. Plant Cell Reports 18: 485-489; Hooker C W, Lott W B and Harrich D. 2001. Inhibitors of human immunodeficiency virus Type 1 reverse transcriptase target distinct phases of early reverse transcription. Journal Virology. 75: 3095-3104).
Spearmint like many other herbs is highly seasonal in nature and has high levels of moisture. In order to preserve this highly perishable biomass source, and make it available year round for extraction, a post-harvest technological treatment of tissue such as drying and/or freezing is required. In general, aromatic herbs and spices are the most sensitive to any post harvest processing including drying or freezing techniques which increase the biological deterioration of tissue. Such treatments results in the loss of volatiles and flavors, changes in the color and texture, and decreases in the nutritional value.
Drying is one of the oldest preservation techniques. Natural drying (drying in the shade) and hot air drying are still the most widely used methods. However, these methods have several disadvantages and limitations; for instance, they require relatively long duration and high temperatures for optimum drying. The contact of dried material with hot air causes rapid degradation of important flavor compounds and nutritional substances, as well as color alteration. Another disadvantage of this method is tissue shrinkage, which results in tissue collapse thereby reducing the available biomass. Freeze-drying is a technique by which material is frozen, and then dehydrated under vacuum; a process by which the contained water passes from a frozen to a gaseous state. Although freeze drying is an excellent method from a quality standpoint, the drying process requires more time and specialized equipment, resulting in high energy and capital costs.
Compared to air drying, hot air drying and freeze drying, microwave or hybrid microwave drying techniques (microwave-hot air drying; microwave-freeze drying, microwave-vacuum drying; osmotic pretreatment before combined microwave-hot air drying) can greatly reduce the drying time of the biological materials while maintaining quality. There has been extensive research into microwave drying techniques, particularly on drying fruits and vegetables (Bouraout M, Richard P and Durance T. 1994. Microwave and convective drying of potato slices, Journal of Food Process Engineering 17: 353-363; Tulasidas T N, Ratti C and Raghavan G S V. 1997. Modelling of microwave drying of grapes, Canadian Agricultural Engineering 39:57-67; Funebo T and Ohlsson T. 1998. Microwave-assisted air dehydration of apple and mushroom, Journal of Food Engineering 38 (3):353-367). The introduction of a microwave drying/heating technique which reduces drying time considerably and produces a high-quality end product offers a promising alternative and a significant contribution to the herb processing industry. Although microwave drying is a rapid technique, the tissues are not evenly dried due to the non-uniformity in temperature distribution (Vadivambal D S and Jayas R. 2007. Changes in the microwave treated agricultural products—a review. Biosystems Engineering. 28:1-16).
Vacuum-microwave drying offers an alternative way to improve the quality of dehydrated products. The low temperature and fast mass transfer conferred by vacuum, combined with rapid energy transfer of microwave heating, generates very rapid, low-temperature drying. Due to the absence of air during drying, the structure, color and sensory qualities of products can be better preserved. Vacuum-microwave drying requires a large capital investment but has been successfully used in the dehydration of fruits and vegetables (Vadivamabal et al.).
In a recent study, different drying techniques such as convective drying, sun drying, room air drying and solar drying (using polythene tent dryer) were carried out in spearmint for comparison of rehydration characteristics, color, oil content and drying ratio (Parminder K, Satish K, Sadhana A, Neena C, and Manpreet S. 2009. Influence of different drying techniques on quality of spearmint (Mentha spicata L.). Journal of Food Science and Technology. 46(5): 440-444). In this study, conventional air dried mint samples had higher oil content compared to convective dried samples, particularly at higher temperatures. Retention of green color was higher in convective dried compared to conventional dried samples at higher temperatures. In a study (Fletcher et al.; 2005b) to understand the effect of heat stress on growing spearmint plants, high temperature drying (80° C.) was found to significantly reduce the total phenolics (up to 87%). This study also indicated that the RA levels were not significantly reduced when tissue was dried at low temperature first (35° C.) followed by high temperature drying.
Several scientific reports exist on improved microwave drying methods for fruits and vegetables in the processing industry (Vadivamabal et al.). However, little information currently exists on direct microwave drying of leafy herbs and among the available reports, as all of them mainly focus on using vacuum-microwave drying technique. The effect of drying methods such as conventional drying, solar drying, oven drying and microwave-vacuum drying on volatile compounds has been investigated in thyme (Thymus vulgaris L.), sage, oregano (Origanum sp), and rosemary (Rosmarinus officinalis L.) (Balladin D A and O. Headley O. 1999. Evaluation of solar dried thyme (Thymus vulgaris L.) herbs, Renewable Energy 17: 523-531); Venskutonis P R, Poll L and Larsen M. 1996. Influence of drying and irradiation on the composition of the volatile compounds of thyme (Thymus vulgaris L.), Flavour and Fragrance Journal 11: 123-128;). All of these studies indicated that higher temperatures and long duration of exposure reduced the levels of the essential volatile compounds in these herbs. No scientific reports exist on drying kinetics of spearmints, especially for the purpose of commercial extraction and stability of phenolic compounds by comparing different drying techniques. The ability to rapidly dry leaf biomass while at the same time maintaining optimal levels of the temperature labile target molecule, rosmarinic acid, would be a major breakthrough in the use of this plant species for phytochemical production.
There is, accordingly, an interest in developing a suitable drying process of spearmint biomass which would retain the highest levels of rosmarinic acid. The ideal drying technique would permit early harvest, lighter weight for transportation from multiple locations and less space for long term storage of biomass without deterioration to allow for year-round extraction.